Brazing material

ABSTRACT

A brazing material for brazing a brazed plate heat exchanger comprising a number of heat exchanger plates being provided with a pressed pattern of ridges and grooves adapted to provide contact points between neighbouring heat exchanger plates, such that the heat exchanger plates are kept on a distance from one another and such that interplate flow channels for media to exchange heat are formed between the heat exchanger plates comprises a brazing alloy comprising at least one melting point depressing element and metals resembling the composition of the heat exchanger plates. The brazing material comprises a mixture between grains of a melting brazing material having solidus and liquidus temperatures lower than a brazing temperature and a non-melting brazing material having solidus and liquidus temperatures above the brazing temperature. The ratio between the melting and non-melting brazing materials is such that an alloy formed by the melting and non-melting brazing materials has a solidus temperature lower than the brazing temperature and a liquidus temperature higher than the brazing temperature.

This application is a National Stage Application of PCT/EP2017/082227,filed 11 Dec. 2017, which claims the benefit of priority to SwedishPatent Application No. 1651661-6, filed 16 Dec. 2016, which applicationsare incorporated herein by reference. To the extent appropriate, a claimof priority is made to each of the above disclosed applications.

TECHNICAL FIELD

The present invention relates to a brazing material for brazing a brazedplate heat exchanger comprising a number of heat exchanger platesprovided with a pressed pattern of ridges and grooves adapted to providecontact points between neighbouring heat exchanger plates, such that theheat exchanger plates are kept on a distance from one another and suchthat interplate flow channels for media to exchange heat are formedbetween the heat exchanger plates. The brazing material comprises abrazing alloy comprising at least one melting point depressing elementand metals resembling the composition of the heat exchanger plates.

PRIOR ART

In the art of brazing, one or more materials are joined to one anotherby applying a brazing material onto contact surfaces of the materials tobe joined. Thereafter, the temperature of the materials to be joined andthe brazing material is increased to a temperature wherein the brazingmaterial is melted whereas the materials to be joined are not.

In the field of brazed plate heat exchangers, the most common basematerials and brazing materials are stainless steel and copper,respectively. This material combination is very beneficial in many ways,but in some cases, it cannot be used. For example, in some countries, itis prohibited to use copper in tap water applications, and if ammonia isused as a refrigerant, copper is excluded due to copper forming watersoluble complexes in the presence of copper and ammonia.

For applications where copper cannot be used, nickel can be used as abrazing material. Nickel as a brazing material has, however, thedrawback of producing brittle brazing joints due to the melting pointdepressants necessary for lowering the melting temperature of nickel.The melting point depressants are usually Silicon, Boron, Phosphorous orcombinations thereof.

Still another option is to use a stainless steel based brazing material,i.e. a stainless steel mixed with melting point depressants such asSilicon, Boron and/or Phosphorous. Using a stainless steel with meltingpoint depressants has, however, the drawback that the melting pointdepressants tend to migrate into the base material, which will lower themelting point of the base material and hence lead to erosion and“burn-throughs”, i.e. a phenomenon caused by the melting pointdepressants in the brazing material lowering the melting point of theplate material to such an extent that the plate material melts over theentire thickness of the plate, hence causing a hole through the plate ofthe heat exchanger plates.

A solution to the erosion problems is presented in the patentapplication SE 1550718-9, filed by the applicant of this application.Here, the amount of melting point depressants is so low that the brazingmaterial does not melt during the brazing. Rather than being joined bythe base material and the brazing material being solving into oneanother during the brazing process, the non-melting brazing material andthe base material will be joined by diffusion. The joining methoddisclosed in SE 1550718-9 is beneficial in that it gives virtually noerosion, but a drawback is that the joint comprising the particles ofnon-melting brazing material becomes porous. The porosity may in somecases lead to the joint being weak.

It is well known that braze alloys containing a melting point depressantsuch as Silicon, Boron and/or Phosphorous does not have a singletemperature melting point. Rather, such braze alloy has a meltingtemperature interval with a lower temperature (the “solidustemperature”) where the braze alloy starts to melt and a highertemperature (the “liquidus temperature”) where the braze alloy iscompletely melted.

It is the object of the present invention to provide a brazing materialthat gives a brazing joint having low or no porosity while erosion ofbase material is low

SUMMARY

The present invention solves, or at least mitigates, the above and otherproblems by providing a brazing material comprising a blend betweengrains of a melting brazing material having solidus and liquidustemperatures lower than a brazing temperature and grains of anon-melting brazing material having solidus and liquidus temperaturesabove the brazing temperature, wherein the ratio between the melting andnon-melting brazing materials is such that an alloy formed by themelting and non-melting brazing materials has a solidus temperaturelower than the brazing temperature and a liquidus temperature higherthan the brazing temperature.

Due to its rapid diffusion rate and high melting point decreasingeffect, the melting brazing material grains may comprise boron as amelting point lowering element.

In order to resemble the composition of the heat exchanger plates, themelting brazing material grains may comprise 1.5-3% Mo, 10-15% Ni,16-20% Cr, 7-10% Si, 5-7% Mn, 1-2% B, balance Fe.

For the same reason, the non-melting brazing material grains maycomprise 1.5-3 Mo, 10-15% Ni, 16-20% Cr, 5-7% Si, 5-7% Mn, balance Fe.

In order to exploit the rapid diffusion of Boron, the compositions ofthe melting and non-melting brazing materials may be identical exceptfor the melting brazing material containing 1-2% Boron.

In order to match a desired brazing temperature, the non-melting brazingmaterial may have a solidus temperature exceeding 1250 degrees C. andthe melting brazing material has a liquidus temperature below 1250degrees C.

In order to reduce erosion of base material while giving a non-porousbrazing joint, an alloy formed of the melting and non-melting brazingmaterials in the proportions of the brazing material may have a solidustemperature lower than 1250 degrees and a liquidus temperature over 1250degrees C.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described by reference to theappended drawings, wherein:

FIG. 1 is a typical DTA-TGA curve for four typical braze alloyscomprising one or more melting point depressants;

FIG. 2 is a micrography of a polished prior art brazing joint wherein abrazing material having solidus and liquidus temperatures lower than thebrazing temperature has been used to join heat exchanger plates;

FIG. 3 is a micrography showing a polished prior art brazing jointwherein a brazing material having solidus and liquidus temperatureshigher than the brazing temperature has been used;

FIG. 4 is a micrography showing a polished brazing joint according to anembodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

In FIG. 1, two DSC (Differential Scanning calorimetry) curves fortypical brazing alloys comprising one or more melting point loweringelements are shown. In short, a DSC curve represents the difference inthe amount of heat required to increase the temperature of a sample andreference is measured as a function of temperature.

With reference to FIG. 1, the upper DSC curve comprises a first segmentfrom 700 degrees to about 1120 degrees C. In this segment, the heatrequired to heat the sample (in this case a brazing alloy) is ratherconstant. From about 1120 degrees C. to 1164 degrees C., i.e. a secondsegment, much more energy is required in order to increase thetemperature. This is due to melting of the braze material. From about1164 degrees C. to about 1180 degrees, i.e. a third segment, the energyrequired to increase the temperature is going back to about the samevalue as during the first segment, and in a fourth segment, the brazingmaterial is completely melted and the energy required for increasing thetemperature will be constant again.

In order to deduce the so-called solidus and liquidus temperatures fromthe curves, i.e. the temperatures where all brazing material is in solidstate and liquid state, respectively, it is common to translate thesetemperatures to onset and offset temperatures respectively, wherein theonset temperature is a temperature of a crossing point between a meantangent of the first segment and a mean tangent of the second segmentand wherein the offset temperature is a temperature of a crossing pointbetween a mean tangent of the third segment and a mean tangent of thefourth segment. In the following, the onset and offset temperatures forthe DSC curves will be regarded as the solidus and liquidus temperaturesof the brazing material.

In the following figures, section photographs of brazing joints will beshown. All of the photographs show brazing joints between crossingridges and grooves of neighbouring heat exchanger plates. The brazingmaterial has been added in form of a paste comprising solvent, a binderand a powder of brazing alloy.

In FIG. 2, a prior art brazing joint between ridges and grooves ofneighbouring heat exchanger plates comprised in a plate heat exchangeris shown. The braze material is an alloy comprising stainless steel andmelting point depressants (boron and silicon), and the braze temperatureis higher than both the liquidus and solidus temperatures of the brazingmaterial—in other words, the brazing material is completely meltedduring the brazing. Also, it could be added that the brazing materialhas been added close to, but not at, the crossing point between theridges and grooves of neighboring plates, such as disclosed inWO2015/062992A1 by the applicant. As can be seen in FIG. 1, the brazingjoint is homogenous (i.e. it comprises no or very few pores). However,at the outskirt of the joint, some erosion of the plate material can beseen. This erosion is due to melting point lowering elements in thebraze alloy migrating into the plate material, hence lowering themelting point of the plate material. Hence, the plate material to whichthe melting point depressing element has migrated might melt and becomepart of the brazing joint, hence reducing the thickness of the platematerial in the vicinity of the brazing joint. In severe cases, theerosion of the plate material might lead to so-called “burn-through”,meaning that there will be a hole through the heat exchanger plate. Thisis, of course, disastrous for a plate heat exchanger, since a holethrough a plate will lead to an internal leakage in the heat exchanger.

In order to reduce the erosion and burn-through problems with suchbrazing materials shown in FIG. 2, the applicant has made experimentswith a brazing material having solidus and liquidus temperatures thatare higher than the brazing temperatures—in other words, the brazingmaterial will not melt during the brazing. Instead of joining the heatexchanger plates by melting and subsequent solidifying, as occurs withthe brazing material of FIG. 2, the brazing material having solidus andliquidus temperatures higher than the brazing temperature will join theplates by diffusion. A typical brazing joint achieved by the brazingmaterial having higher liquidus and solidus temperatures than thebrazing temperature is shown in FIG. 3. As can be seen in this figure,the brazing joint is completely free from erosion—which is notsurprising, since the amount of melting point depressants in the brazingmaterial is not sufficient to enable melting of the brazing materialitself. In case some of the melting point lowering elements wouldmigrate into the plate material, the percentage of melting pointlowering elements in the plate material also would not be sufficient tomelt the plate material. It could be mentioned that, in contrast to FIG.2, the brazing material has been applied at the contact point betweenridges and grooves of neighbouring plates.

It could also be seen that a central portion of the brazing joint isfree from pores; the pressure between crossing ridges and grooves ofneighbouring plates is sufficient to make particles of the brazingmaterial fuse together and form a pore free joint. In the outskirts ofthe brazing joint, there are, however, pores.

Tests have shown that the strength of the brazing joints of FIGS. 2 and3 are about equal in terms of N/mm², but since the effective, i.e. porefree, joint achieved with the brazing material of FIG. 3 is smaller, theresulting joint strength is lower than for the joints achieved accordingto FIG. 2. On the other hand, there is no erosion.

As mentioned in the “Summary”-section, it is the object to provide abrazing material giving pore free joints and no or low erosion of thebase material, i.e. the plate material.

This is achieved by mixing a powder of a non-melting brazing alloy, i.e.an alloy having higher solidus and liquidus temperature than the brazingtemperature, and a melting brazing alloy, i.e. a brazing material havinglower liquidus and solidus temperatures than the brazing temperature.

Common for all embodiments is that more than 50% of the brazing alloy(by weight) is a non-melting brazing alloy. Also common for allembodiments is that the brazing material is in form of a paste, and thatthe paste comprises grains of brazing material, wherein a certainpercentage of the grains are manufactured from a melting brazingmaterial and the remainder of the grains are manufactured fromnon-melting brazing material. Except from the grains of brazingmaterial, the paste also comprises a solvent and a binder serving toform a paste of the grains.

Typical grain sizes for the brazing materials are less than 100 microns.This means that the total surface area of the grains will be many timeslarger than the surface area of the base material the brazing materialcovers.

As mentioned earlier, the major problem with melting brazing materialscomprising melting point lowering element(s) is that the melting pointlowering element(s) tend to migrate into the base material and lower themelting point thereof such that the base material melts, which causeserosion of the base material. The erosion process is very dependent ontime—the longer brazing times, the more erosion. According to anembodiment, however, the time available for the melting point loweringelements to migrate into the base material is controlled in an efficientmanner: once the melting brazing material has melted, the melting pointlowering elements will start to migrate, but since the surface area ofthe non-melting grains are so much larger than the area of the basematerial that is in contact with the brazing material, a majority of themelting point depressant will migrate into the non-melting brazingmaterial grains rather than into the base material. Of course, this willlead to “erosion” of the non-melting grains, but it will also lead tothe melted brazing material being depleted of brazing material, meaningthat when all or a majority of the non-melting brazing material has been“eroded” or dissolved, there will be not enough melting point depressantleft in the melted brazing material to migrate into the base material tothe extent that the melting point thereof will become lower than thebrazing temperature. Hence, erosion of the base material is mainlyavoided.

As an example, the non-melting brazing material grains could be madefrom pure stainless steel of the same type as the base material.However, the best results have been achieved by providing thenon-melting brazing material in form of grains made from stainless steelmixed with a small amount of melting point depressing elements. This isprobably due to faster depletion/dilution of the melting brazingmaterial.

Tests have shown that a mixture of non-melting brazing material grainscomprising (by weight) 2% Mo, 12.5% Ni, 18.2% Cr, 6.3% Si, 5% Mn,balance Fe and melting brazing material grains comprising (by weight)2.1% Mo, 13.4% Ni, 18.4% Cr, 8% Si, 5.3% Mn, 1.1% B, balance Fe, in theproportions 90-75/10-25 will produce strong and pore free joints givingno or very little erosion of a base material made from 316 stainlesssteel when brazed at a temperature of 1250 degrees C. under anatmosphere of 1-1.5 mbar inert gas.

The non-melting brazing material according to the above has a solidustemperature of about 1260 degrees C. and a liquidus temperature of about1370 degrees C., i.e. both the solidus and liquidus temperatures areabove the brazing temperature of 1250 degrees C.

The melting brazing material has a solidus temperature of about 1110degrees C. and a liquidus temperature of about 1195 degrees C., i.e.both the solidus temperature and the liquidus temperatures are lowerthan the brazing temperature.

Using a simple interpolating procedure, the solidus and liquidustemperatures of an alloy made from brazing material containing 10%melting brazing material and 90% non-melting brazing material accordingto the above is estimated to have a solidus temperature of 1245 degreesC. and a liquidus temperature of 1352 degrees C., and an alloycomprising 25% melting brazing material and 75% non-melting brazingmaterial is estimated to have a solidus temperature of 1222 degrees C.and a liquidus temperature of 1326 degrees C. In other words, thesolidus and liquidus temperatures of both the alloys will be such thatthe solidus temperature is lower than the brazing temperature and theliquidus temperature is higher than the brazing temperature.

By doing the same interpolation between the solidus and liquidustemperatures of a non-melting brazing material comprising grains of purestainless steel and melting brazing material according to the above,similar solidus and liquidus temperatures as disclosed above for theresulting alloy (i.e. solidus and liquidus temperatures being lower andhigher than the brazing time, respectively) for a melting brazingmaterial content of 27% to 53%.

The time required to form a homogenous alloy from the grains of meltingand non-melting brazing material grains is highly dependent on the grainsize of, primarily, the non-melting brazing material grains; smallergrains will give shorter time for forming a homogenous alloy.

Using a non-melting brazing material made from stainless steel grains iseconomically advantageous, since such grains can be bought“off-the-shelf” in various grain sixes, whereas brazing material grainsmade from melting brazing material can not.

The present invention is particularly for brazing with brazing materialcomprising melting point depressing elements that can migrate into thebase material, for example stainless steel base material brazed with astainless steel braze material comprising melting point loweringelements in form of Silicon, Boron, Phosphorous and/or Manganese. Ofthese melting point depressing elements, Boron is the most efficient interms of melting point lowering effect as a function of percentage, andit is also the element with the fastest diffusion rate into the basematerial or neighbouring non-melting grains having a lower percentage ofBoron. It also tends to form brittle Borides, predominantly with theChromium in the stainless steel. Except from the Borides being extremelybrittle, the formation of Chromium borides “consume” considerableamounts of chromium, which after the formation of boride will not givethe corrosion resistance properties it is supposed to. Chromium boridesare predominantly formed in the liquid phase, and since the presentinvention reduces the time the brazing material is in liquid phase, thepresent invention will reduce the problems with boride formationsignificantly, hence improving both brazing joint strength and corrosionresistance of the joint.

Silicon and Phosphorous are less efficient melting point loweringelements in terms of melting point lowering effect as a function ofpercentage. Therefore, larger amounts of these melt depressants areneeded, and they are therefore equally problematic in terms of brittlephase formation.

Manganese is a melting point lowering element that except from themelting point lowering effect also provides for a good wetting betweenthe brazing material and the base material. Manganese, however, tends tovaporize during the brazing (especially if the brazing is performedunder a vacuum), meaning that the melting temperature of the finishedbrazing joint might be higher than the initial melting of the brazingmaterial. This is usually no problem, but if unnecessarily large amountsof manganese are used, there might be a problem with brazing furnacefouling.

Preferably, the brazing material is provided in the form of a pastecomprising melting and non-melting brazing material particles, a solventfor providing rheological properties to the paste for its applicationand a binder adapted to keep the brazing material particle together whenthe solvent has evaporated. However, other possibilities are alsopossible. As an option, the brazing material may be provided as a greenbody, i.e. a body comprising powdered melting and non-melting brazingmaterial particles that has been pressurized such that a cold sinteringoccurs between the particles. It should be noted that, according to theinvention, the green body should not be heat treated such that thecomposition differences of the brazing material grains is equalizedprior to the green-body being used as brazing material in the brazingprocess.

The invention claimed is:
 1. Brazing material for brazing a brazed plateheat exchanger comprising a number of heat exchanger plates beingprovided with a pressed pattern of ridges and grooves adapted to providecontact points between neighboring heat exchanger plates, such that theheat exchanger plates are kept on a distance from one another and suchthat interplate flow channels for media to exchange heat are formedbetween the heat exchanger plates, the brazing material comprising abrazing alloy comprising at least one melting point depressing elementand metals resembling a composition of the heat exchanger plates,wherein the brazing material comprises a mixture of grains of a meltingbrazing material having solidus and liquidus temperatures lower than abrazing temperature and a non-melting brazing material having solidusand liquidus temperatures above the brazing temperature, wherein a ratiobetween the melting and non-melting brazing materials is such that analloy formed by the melting and non-melting brazing materials has asolidus temperature lower than the brazing temperature and a liquidustemperature higher than the brazing temperature, wherein more than 50%by weight of the brazing alloy is non-melting brazing material, andwherein the non-melting brazing material is made from stainless steelmixed with melting point depressing elements.
 2. The brazing material ofclaim 1, wherein the melting brazing material grains comprise boron as amelting point lowering element.
 3. The brazing material of claim 2,wherein the melting brazing material grains comprise 1.5-3% by weightMo, 10-15% by weight Ni, 16-20% by weight Cr, 7-10% by weight Si, 5-7%by weight Mn, 1-2% by weight B, balance Fe.
 4. The brazing materialaccording to claim 1, wherein the non-melting brazing material grainscomprise 1.5-3% by weight Mo, 10-15% by weight Ni, 16-20% by weight Cr,5-7% by weight Si, 5-7% by weight Mn, balance Fe.
 5. The brazingmaterial according to claim 1, wherein compositions of the melting andnon-melting brazing materials are identical except for the meltingbrazing material containing 1-2% by weight boron.
 6. The brazingmaterial according to claim 1, wherein the non-melting brazing materialhas a solidus temperature exceeding 1250 degrees C.
 7. The brazingmaterial according to claim 1, wherein the melting brazing material hasa liquidus temperature below 1250 degrees C.
 8. The brazing materialaccording to claim 1, wherein an alloy formed of the melting andnon-melting brazing materials in the proportions of the brazing materialhas a solidus temperature lower than 1250 degrees C. and a liquidustemperature over 1250 degrees C.